Because watercolor is water, and water always runs downhill, your paper tilt is another tool for you to get to know. Just like your brushes and paints, the angle at which you’re painting is important.
Some people like to paint flat, others vertical, and the rest of us paint at a tilt. Whatever you do is fine, it just depends on what you want.
For a reference, I made three little examples for you to look at:
And on closer inspection…
This one was painted and dried flat.
You can see how the pigment stayed relatively even, and coalesced around the edges a bit. There’s a very slight bloom in the middle, as the edges dried faster than the center, but in general the pigment is dispersed and moving evenly.
This one was painted and dried at a tilt.
The second was left to dry at a very slight incline (about 15 degrees). You can see how it pooled at the bottom, and some of the pigment was carried back up by the standing water.
Some people consider this effect to be desirable, some people will give you a bad grade for it. Either way, just be aware that if you leave water to dry at a tilt, something like this will happen. You can use this to your advantage, and analyze a puddle on your page and decide where you want it to pool, and rotate your painting accordingly. Neat, huh?
This one was painted and dried (nearly) vertical.
The last one was painted and left to dry at a near vertical slant. Vertical is a hard angle for watercolor, and frankly I rarely paint this way. But, I’ve seen some incredible artists who do. You have to use much less water and build up layers of color patiently. Too much water will drip immediately. But done well, the effects are clean and even. I was a little impatient with this, and water still pooled a tad at the bottom. Oh well, you get the idea.
For me, I tend to paint on a very slight incline. Totally flat is fine too, but I usually end up propping up one end of my painting on a book at some point. I like using gravity to direct the paint, and I personally like the way watercolor looks with some controlled blooming.
But to each their own.
So there you have it, one more tool for your box: painting incline.